Our long-range objective is to develop a clearer fundamental understanding of the molecular basis of fertilization so that the process can be controlled to either promote fertilization or block it. Much progress has been made to identify ZP3, the protein in the mammalian egg coat that binds sperm and stimulates the acrosome reaction, the release of the acrosomal vesicle that must occur for sperm to penetrate the egg coat. Despite this progress, the identity of the ZP3 receptor on sperm has been elusive. One strong candidate is beta1,4-galactosyltransferase, an enzyme first discovered in the Golgi apparatus of somatic cells but later found on the surface of sperm. Beta1,4-Galactosyltransferase binds ZP3 but not other egg coat proteins. In addition to its role in gamete adhesion, beta1,4- galactosyltransferase acts to trigger the acrosome reaction in sperm. The acrosome reaction can be stimulated with antibodies to beta1,4-galactosyltransferase that act as mimics of ZP3 to bind and crosslink beta1,4-galactosyltransferase. There is evidence that binding beta1,4-galactosyltransferase stimulates at least some of the same signaling systems inside sperm that ZP3 stimulates, leading to the proposal that beta1,4- galactosyltransferase may be the major ZP3 receptor signaling the acrosome reaction. Mice with a targeted deletion of the beta1,4-galactosyltransferase gene, although fertile, produce sperm with severely compromised ability to acrosome react and penetrate the egg coat. In this proposal, the signaling systems that are activated by beta1,4-galactosyltransferase will be studied. The goals are to determine if binding beta1,4-galactosyltransferase specifically can completely reproduce all of the identified signaling systems activated by ZP3 binding, to study beta1,4-galactosyltransferase mutants to identify structural features necessary for signaling, and to identify proteins that interact with the portion of beta1,4- galactosyltransferase found in the cytosol. These studies will clarify the role of a sperm receptor for the egg coat in the acrosome reaction, a necessary step in the process of fertilization.